Guar gum is widely used in the oil and gas industry in various well treatment procedures to increase production of oil and gas from a well. The general objective is, of course, to obtain oil and gas as a valuable commodity. Oil and gas is used in the production of products such as refined fuels and oils, and it is used also as the raw material for many types of plastics and chemicals.
Typically, oil and gas is found accumulated and trapped in various subterranean formations. The formations are considered to be subterranean regardless of whether they are under land or under water. For example, an oil and gas bearing subterranean formation may be offshore under a sea floor. To reach an oil and gas bearing subterranean formation, it is almost always necessary to drill a well many thousands of feet into the earth.
Drilling a well to reach one or more oil and gas bearing subterranean formations is merely one of the many challenges to bringing the oil and gas up to the surface. Another challenge is extracting the oil and gas from the subterranean formation. For example, the oil and gas may be trapped in the tiny pores in the rock of the subterranean formation, and the interconnections between the pores may be so few or poorly interconnected that it is difficult for the oil and gas to flow through the rock and into the well. Sometimes the formation is damaged by water being introduced into the formation by the mere drilling of the well into the formation, or by the water present in the drilling fluid used to drill the wellbore. Sometimes the formation is damaged by the migration of certain types of hydrocarbon, such as waxes. Sometimes the formation is damaged by the movement of tiny particles called “fines” that plug the interconnections between the pores in the rock. Thus, the permeability of the subterranean formation to the fluid flow of oil and gas is often very low. This presents another challenge to increase the flow of oil and gas through the rock of the subterranean formation and into the well. In the oil and gas industry, treatments performed to restore or enhance the productivity of a formation are referred to as “stimulation.”
Of the various stimulation techniques, one of the most common and widely accepted is hydraulic fracturing. In general, hydraulic fracturing involves injecting a fracturing fluid through the wellbore and into an oil and gas bearing subterranean formation at a sufficiently high rate of fluid flow and at a sufficiently high pressure to initiate and extend one or more fractures in the formation. To conduct hydraulic pressure through the wellbore, the fracturing fluid must be substantially incompressible. In addition, because of the large quantities of fracturing fluid required, the fracturing fluid is preferably based on readily-available and plentiful fluid. Thus, the typical fracturing fluid is based on water.
The fracturing fluid is injected through the wellbore at such a high flow rate and under such high pressure that the rock of the subterranean formation that is subjected to the hydraulic treatment cracks apart or fractures under the strain. When the formation fractures, the pressure is relieved as the fracturing fluid starts to move quickly through the fracture and out into the formation. The theoretical objective of forming such a fracture in the rock of the formation is to create a large surface area of the faces of the fracture. The large surface area allows oil and gas to flow from the rock of the subterranean formation into the facture, which provides an easy path for the oil and gas to easily flow into the well.
However, once the high pressure is relieved suddenly by the escape of the fracturing fluid through the created fracture and out further into the subterranean formation, the fracture has a tendency to be squeezed closed by the natural pressures on the rock within the deep subterranean formation. To keep the fracture open, some kind of material must be placed in the fracture to prop the faces of the fracture apart.
The desirable material for the purpose of propping the fracture apart must meet several criteria. For example, the material must have a sufficient strength not to be entirely crushed by the natural forces tending to push the fracture closed. The material must be capable of being fluidized so that it can flow with or immediately following the fracturing fluid. Additionally, the material also must itself not block or seal the fracture. Thus, a typical material used for the purpose of propping open a fracture is sand. Sand, in the aggregate, has a sufficiently high mechanical strength to prop open a fracture in a subterranean formation at typical depths and natural subterranean pressures; it can behave as a fluid in that it can be poured and flow; and the particles, even when tightly compacted, have a network of void spaces between them that can provide high porosity to fluid flow.
While sand is the most commonly used material for the purpose of propping the fracture open, many other materials of the appropriate size range and mechanical strength can be used. In the oil and gas industry, any suitable particulate material that is used for the purpose of propping open a fracture produced by hydraulic fracturing is called a “proppant.”
To be able to carry and place a proppant into a newly-created fracture, a fluid must have a sufficient viscosity to suspend and carry the proppant. In a low viscosity fluid, for example, the proppant would have a tendency to simply fall under gravity toward the bottom of the well instead of being carried with the fracturing fluid out into the newly-created fracture. For a fluid to be able to carry the proppant instead of having the proppant fall out of the fluid, the fracturing fluid needs to be made to have a much higher viscosity than that of water. Preferably, the fracturing fluid is a gel, which has a very high viscosity and great capacity for carrying a proppant suspended in the fluid.
Using a water-soluble gum is one of the ways to build viscosity in aqueous systems. Such a gum can be mixed with an aqueous fluid for use in a well to increase fluid viscosity. A sufficient concentration of the gum in an aqueous system can form a linear gel. Furthermore, the gum also can be crosslinked with other compounds, such as borates or various metals, to create a highly viscoelastic fluid, which is highly advantageous to transporting a proppant in a hydraulic fracturing procedure.
In the oil and gas industry, the gum conveniently is obtained in the form of a powder. The powder also can be suspended conveniently in a non-aqueous fluid, such as diesel, because the gum will not dissolve or swell with a non-aqueous fluid and being suspend in a non-aqueous fluid allows the gum powder to be handled as a liquid.
The oil and gas industry currently uses millions of pounds of gum per year to help build viscosity in aqueous systems, including for use in stimulation procedures such as hydraulic fracturing. The driving factor in selecting a source of gum for use in the oil and gas industry is cost.
Gum is found in certain seeds of Leguminosae, such as the seeds of the guar plant, the carob tree, the honey locust tree, and the flame tree. Among the available agricultural sources of gum, guar seed is one of the most economical.
The guar plant is drought resistant. The guar plant can be grown economically in semiarid regions of the world, such as India and Pakistan, where few other types of crops are viable. The guar plant grows about three to six feet in height and bears many beanlike pods, each which contains six to nine small, rounded guar seeds. In addition to being hardy, typical varieties of guar seed have a relatively high concentration of gum compared to gum-bearing seed from other Leguminosae.
The guar seed is composed of a germ (or embryo), a pair of endosperm sections, and a husk. The germ is brittle and relatively small compared to the endosperm sections. The germ is sandwiched between the pair of endosperm sections but easily separable from the endosperm sections. The endosperm sections contain mostly water-soluble gum (i.e., galactomannan polysaccharide) and minor amounts of proteinaceous material, inorganic salts, water insoluble gum, and cell membranes. The endosperm sections are tough and non-brittle. The endosperm sections are enclosed in the husk, which also is often referred to as the hull. The husk is very tough and very tightly associated with the endosperm sections. Typical varieties of guar seed have about 40%-46% by weight of the germ, about 38%-45% by weight of the endosperm sections (containing the gum), and about 14%-20% of the husk. Unless otherwise stated, all percentages are by weight, and, unless the context otherwise requires, on a dry basis.
However, processing guar seed into gum is mechanically challenging. In particular, it is difficult to separate the gum-containing endospern sections from the husk selectively.
The conventional process for extracting gum from guar seed includes splitting the seed, which splitting step is often termed “seed processing.” In splitting the seed, germ and undehusked guar splits are obtained. The undehusked guar splits are the endosperm sections with the husk still on them. The undehusked guar splits are extremely tough. As part of the splitting step, the brittle, relatively small germ is easily and substantially separated from the relatively large undehusked guar splits by particle size screening.
Next, the undehusked guar splits are processed conventionally in an attempt to separate the endosperm sections and the husk. This is done conventionally by passing the undehusked guar splits through an extruder having an elongated cylindrical barrel provided with an inlet opening, a die opening at the outlet end of the barrel, and a screw rotatable within the barrel to transport and work the undehusked guar splits. This step is sometimes referred to as “dehusking” or “dehulling” the undehusked guar splits. This step produces a product referred to as “dehusked guar splits” and “husk.” As part of the dehusking step, the dehusked guar splits and husk are separated by particle size screening.
The “dehusked guar splits” obtained from the conventional dehusking of undehusked guar splits are relatively large, mostly substantially in the form and shape of the endosperm sections, but still having bits of husk still attached to them. Thus, the “dehusked gaur splits” are actually not completely dehusked. Typically, dehusked guar splits comprise about 90% endosperm sections (w/w dry basis) wherein the husk has been substantially but not completely removed. Typically, the dehusked guar splits still contain about 10% husk material.
The “husk” obtained as a by-product from the conventional dehusking of undehusked guar splits is relatively small bits and pieces of husk material of various sizes and shapes. But, in the dehusking step, some of the endosperm material has also been removed with the husk and pieces of the endosperm material have been broken off from the endosperm sections. Thus, the “husk” is not only husk material, but includes bits and pieces of the endosperm sections. Typically, the “husk” comprises about 25-40% endosperm (containing the valuable gum), usually in the form of broken off pieces from the endosperm sections.
For example, after starting with a given amount of typical guar seed, in the splitting step, about 30% by weight is removed as germ. Then in the dehusking step, as much as an additional 37% by weight of the original amount of guar seed is removed as “husk.” The husk from the guar seed, however, includes substantial amounts of the endosperm. Typically, the husk comprises in the range of about 25% to about 40% by weight gum, and most typically about 30-35% by weight gum. Thus, the dehusking step removes about 10% to about 30% (typically 25%) by weight of the gum in the original amount of typical guar seed. Nevertheless, because of the difficulty in separation and the relatively low overall concentration of the endosperm material (i.e., gum) contained with the removed husk (i.e., less than about 30%), the husk obtained from conventional guar seed processing has low economic value and is usually considered to be a waste by-product, being typically sold for animal feed.
The dehusked guar splits are then flaked and ground into a powder. The dehusked splits are normally soaked in water from 30 minutes to several hours, at concentrations ranging from 80% to 120% by weight of splits relative to the amount of water, with 100% to 110% being preferred. Flaking is then accomplished by passing the wet dehusked guar splits between two counter rotating rollers, one roller moving substantially faster than the other, thus creating high levels of shear causing the wet splits to shred into moist fibers or flakes. Usually this process imparts such significant mechanical energy that part of the moisture is evaporated and the fiber/flakes heat up several degrees. The flakes are then passed through a hot air grinding or a hammer mill to reduce the flakes to a powder. Hot air is used to transport the flakes into the grinder, as well as to “flash dry cool” the particles during grinding. The temperature, humidity and mixture ratio of the air and flakes are such that the evaporative cooling during grinding offsets the heat generated from grinding so that it prevents the flakes and subsequently the ground powder from exceeding some designated maximum temperature. The powder typically ranges from about 10 microns to about 100 microns in average particle size. The powder is typically less than 10% water by weight and has a very stable and long shelf life. As previously mentioned, the guar gum powder is sometimes suspended in a non-aqueous fluid, such as diesel or environmentally friendly hydrocarbons, for storage and ease of handling.
When desired to be used, the powder is dispersed in water and allowed to hydrate. The viscosity of the solution depends in part on the concentration of the gum, usually in the range of about 0.05% to about 5% by weight of water, where 0.1% to 0.5% by weight of water, is the preferred range.
Despite the challenges in processing the gum from guar seed and the waste, guar gum is still one of the most common means with which to build viscosity in aqueous systems, especially for fracturing fluids (see, for example, U.S. Pat. Nos. 2,891,050 and 3,455,899). In general, it would be commercially valuable to be able to use more of the guar gum from guar seeds than conventional processing has allowed, including for applications outside the oil and gas industry.